LONDON — Britain’s intelligence agencies missed chances to thwart last year’s transit attacks by failing to follow up leads on two of the men who became the country’s first suicide bombers, major reports said Thursday.

The government blamed a lack of funds, a too-slow buildup of intelligence staff in the wake of the Sept. 11 attacks on the United States, and spies’ failure to anticipate that British citizens would contemplate suicide attacks on their homeland.

But the reports found “no culpable failures” by agencies, including the MI5 and MI6 intelligence services, saying the bombings of three London subways and a double-decker bus July 7 came without warning.

Britain’s Home Office said in one of the reports that there is “as yet no firm evidence” of al-Qaida’s role, if any, in organizing the attacks, which killed 52 commuters and the four bombers.

However, suspected ringleader Mohammed Sidique Khan and accomplice Shezad Tanweer traveled to Pakistan and it is “likely that they had some contact with al-Qaida figures,” said a second report, by the Intelligence and Security Committee, a panel of nine British lawmakers.

In September, Khan made a posthumous farewell in a videotape aired on Al-Jazeera television. Khan said he was inspired by al-Qaida leader Osama bin Laden, his deputy Ayman al-Zawahri and by Abu-Musab al-Zarqawi.

The association of the al-Qaida leader and the 30-year-old suicide bomber was considered at the time to be the strongest link yet of a role by the terror organization in the attacks. The Home office report Thursday said the inquiry was continuing.

No links have been found between the July 7 bombers and the group that mounted failed bombing attempts against the transport system two weeks later, one of the reports said.

Survivor groups were unsatisfied with the probes, demanding that the government conduct a public inquiry such as the one in the United States after Sept. 11, 2001.

“It’s a whitewash,” said Rachel North, 35, who survived the subway car bombing close to Russell Square station, which killed 26 people. “Reports written in secret by people hoping to save their own jobs usually find there is no one to blame. They only add to our need for a full public inquiry.”

Many Britons had initially been stunned to learn the attackers were compatriots — three of Pakistani descent and one originally from Jamaica.

Britain’s intelligence and law enforcement communities were also taken by surprise, the committee report said.

Rather than tracking two men who became bombers — originally thought to be involved in fundraising — resources were understandably shifted to focus on “known plans” to attack the United Kingdom, the report said.

However, the report said authorities would have had a better chance of preventing the bombings had they channeled more resources into intelligence work in Pakistan and grasped the potential for Britons to strike against their countrymen.

There was at least one “missed opportunity” to investigate Khan in two intelligence operations in 2003 and 2004, the committee report said.

It said Khan and Tanweer were “on the peripheries” of an “important and substantial,” inquiry into another plot in 2004. But authorities believed they were trying to raise money for radicals rather than training for an attack.

“As there were more pressing priorities at the time, including the need to disrupt known plans to attack the U.K., it was decided not to investigate them further or seek to identify them,” the report said.

Intelligence agencies also knew Khan had planned meetings with a terrorist suspect in 2003, the report said. It reported for the first time that phone numbers for Khan and a third bomber, Jermaine Lindsay, 19, a Jamaican-born Briton, were also stored in intelligence records.

Home Secretary John Reid, publishing the Home Office summary of investigators’ findings so far, said it appeared the attackers were motivated by “fierce antagonism to perceived injustices by the West against Muslims.”

But despite Khan’s praise of al-Zarqawi in the video, Reid said there was no evidence the 2003 U.S.-led invasion of Iraq was “in any way involved” in the decision to attack Britain.

Investigations are continuing into contacts the men had with individuals in Pakistan before the bombings, Reid said, adding that intelligence officials have discounted claims a foreign terrorist acted as a mastermind.

Eliza Manningham-Buller, head of MI5 — Britain’s internal spy agency — told the committee her agency had been surprised the attackers used the tactic of suicide bombing, the report said.

“Neither the potential speed of radicalization nor the fact that British citizens could be radicalized to the point of suicide were understood” by security services before the attacks, said Paul Murphy, chairman of the intelligence committee.

The group constructed their bombs using readily available materials, with “little expertise” and at a cost of less than $15,000, Reid said.

Authorities had ruled out the possibility there was to have been a fifth bomber — concluding explosives left in a parked car in Luton, north of London, were for use “in case of interception,” Reid said.

Reid said police and intelligence agencies had thwarted three attacks since July 7, but gave no details. Officials urged lawmakers to offer intelligence agencies greater resources and said the threat of more attacks remains potent.

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